A sagittal saw blade is a surgical saw with a head that pivots around an axis that is perpendicular to the blade. PCT Pub. No. WO 2006/017066A2/U.S. Pat. No. 7,497,860 and PCT Pub. No. WO 2007/030793A2/U.S. Pat. No. 7,704,254, the contents of which are each incorporated herein by reference, each disclose a sagittal saw blade cartridge. A sagittal saw blade cartridge includes a static guide bar and a blade head. The guide bar is an elongated member that is releasably attached to the handpiece, the saw that actuates the cartridge. The blade head is pivotally mounted to the guide bar and has teeth that extend forward from the guide bar. One or more drive links extend from the blade head towards the proximal end of the guide bar. The drive links are reciprocated back and forth by a drive assembly internal to the saw. The reciprocation of the drive links causes the blade head to pivot back and forth. The pivoting of the blade head is what enables the teeth to cut the tissue against which the blade head is pressed. Sometimes, this type of cartridge is referred to as an oscillating tip saw blade cartridge.
An advantage of the sagittal blade cartridge is that the only portion of the cartridge that pivots is the distally located blade head. By way of comparison, a conventional sagittal saw blade pivots from its point of attachment to the saw to which the blade is attached. A cartridge, when actuated, vibrates less in the hands of the surgeon holding the handpiece. Also, it is common practice to use a cutting guide to properly position a sagittal saw blade relative to the tissue the blade is intended to cut. When a conventional blade is actuated, the oscillating movement of the blade imposes significant wear on the surfaces of the cutting guide defining the slot in which the blade is seated. The guide bar of a surgical sagittal blade cartridge only minimally moves in this slot. Thus, by using a cartridge, instead of a conventional blade, less of the material forming the cutting guide is rubbed off the guide. This reduces the extent to which the surgeon has to flush worn off cutting guide material from the surgical site. Further, use of the oscillating tip blade reduces the extent to which the material forming the guide becomes so worn that the guide itself is rendered useless.
A surgical sagittal blade cartridge has another benefit over a conventional sagittal saw blade. Since the guide bar remains static when the blade head is actuated, the practitioner can press a finger or two against the guide bar. This facilitates the holding of the cartridge in the desired position as the cartridge is advanced against the tissue that is to be cut.
When either a conventional sagittal blade or a sagittal blade cartridge advances through bone, the blade head is exposed to resistance. This resistance can be appreciable when the cut has a depth of 5 cm or more. Often the bone located adjacent the underside of the cartridge is more resistive to cutting than the bone located immediately above the cartridge. The cartridge, like most mechanical devices, will when advanced forward, advance along the path of least resistance. Since the bone above the cartridge can be less resistance to cutting than the bone below the cartridge, a cartridge when advanced, can flex upwardly out of the plane of the desired cut. This upwardly flexing of the blade is known as skiving. There can also be situations when owing to the density of the bone, the cartridge, when advanced, flexes below the plane of the cut. This type of flexure is known as diving.
Regardless of the direction the blade flexes, the flexure is undesirable. This is because a sagittal saw blade cartridge is typically used to remove bone so an artificial implant can be fitted in the space previously occupied by the removed bone. An implant is formed with surfaces designed to precisely seat against the complementary surfaces of the bone against which the implant is mounted. If the cut does not leave the bone with surfaces that have the desired shape, the results of the implant fitting procedure may be less than optimal.
In theory, one could increase the rigidity of a surgical sagittal blade cartridge by increasing the thickness of the guide bar. It should be appreciated that the slots of cutting guide through which the cartridge is inserted tend to be relatively narrow. Often the height this slots is around 1.5 mm or less. This height limit imposes a limit of the thickness of the cartridge guide bar that can be inserted in this slot. Furthermore, if thickness of the guide bar is increased, by extension it is necessary to increase the thickness of the cut that will be formed by the cartridge. Increasing cut thickness can also lead to the cartridge leaving a cut surface that does not have the desired degree of planar smoothness. Increasing guide bar thickness is therefore typically not a viable solution for reducing the incidence of cartridge flexure.